Semiconductor laser device and method for fabricating thereof

ABSTRACT

A semiconductor laser device having on a compound semiconductor substrate at least a lower cladding layer, an active layer, an upper cladding layer and a contact layer is provided. An upper part of the upper cladding layer and the contact layer are formed as a mesa-structured portion having a ridge stripe pattern, and the both sides of the mesa structured portion are buried with a current blocking layer. The laser device comprises the current blocking layer having a pit-like recess penetrating thereof and extending towards the compound semiconductor substrate, and a portion of the recess other than that penetrating the current blocking layer being covered or buried with an insulating film or a compound semiconductor layer with a high resistivity. The compound semiconductor substrate and the electrode layer thus can be kept insulated in an area other than a current injection area, thereby non-emissive failure due to short-circuit is prevented.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a semiconductor laser device anda method for fabricating thereof, and in more detail a semiconductorlaser device having a structure capable of preventing non-emissivefailure due to short circuit and a method for fabricating such device.

[0003] 2. Description of the Related Art

[0004] A visible light semiconductor laser device having a stackedstructure on a GaAs substrate, wherein an active layer is sandwiched bycladding layers made of AlGaInP or GaInP, has an oscillation wavelengthbetween 630 nm and 690 nm, and attracts a good deal of attention as alight source for an optical pickup used in an optical disc drive.

[0005] A structure and fabrication method of a conventional AlGaInP-basevisible light semiconductor laser device will be explained hereinafterreferring to FIG. 5. FIG. 5 shows a cross-sectional view of thesubstrate showing a structure of an AlGaInP-base semiconductor laserdevice.

[0006] An AlGaInP-base semiconductor laser device 10 has on a GaAssubstrate 12 a stacked structure comprises a lower cladding layer 14made of n-AlGaInP, an active layer, an upper cladding layer 18 made ofp-AlGaInP, and a contact layer 20 made of p-GaAs, and all layers areepitaxially grown in this order.

[0007] An additional semiconductor layer such as light confining layermay optionally be provided between the upper cladding layer 18 and thecontact layer 20. Also a buffer layer made of compound semiconductor mayoptionally be provided between the GaAs substrate 12 and the lowercladding layer 14.

[0008] Of such stacked structure, the upper cladding layer 18 and thecontact layer 20 are formed as a mesa-structured portion having a ridgestripe pattern.

[0009] The both sides of the upper cladding layer 18 and the contactlayer 20 composing the mesa-structured portion, and the upper claddinglayer 18 are buried with an n-GaAs layer 22 provided as a currentblocking layer to ensure current constriction, thereby a central portionof the active layer becomes an oscillation area 15 of laser light.

[0010] A metal layer made of Au, Ni and the like, or a metal stackedfilm is provided as a p-side electrode 24 on the n-GaAs layer 22 and thecontact layer 20, and as an n-side electrode 26 on the rear surface ofthe GaAs substrate 12, respectively.

[0011] In order to fabricate such semiconductor laser device 10, atfirst the lower cladding layer 14, active layer 16, upper cladding layer18 and contact layer 20 are epitaxially grown in this order on the GaAssubstrate 12 by the metal-organic chemical vapor deposition (MOCVD)process.

[0012] The contact layer 20 and the upper cladding layer 18 are thenetched to form the mesa-structured portion, and the n-GaAs layer 22 isthen selectively grown on the both sides of the mesa-structured portionand on the upper cladding layer 18.

[0013] Next, the p-side electrode 24 and n-side electrode 26 are formedby, for example, the sputtering process on the outermost surface and onthe rear surface of the GaAs substrate 12.

[0014] In the process of epitaxially growing the AlGaInP layer and thelike to form the stack-structured portion, there has, however, been aproblem of generating a growth defect in the epitaxially grown layer(s)if fine particles of GaAs or so adhere thereon, or foreign intermediateproducts are formed on the substrate during the epitaxial growth.

[0015] In the process of etching the stack-structured portion to formthe mesa-structured portion after the epitaxial growth, etching with anacid of such epitaxially grown layer having the growth defect willresult in formation of a pit-like shape defect portion 28 of several totens μm diameter reaching the GaAs substrate 12 as shown in FIG. 6,since the portion of the growth defect is labile to acid and shows ahigh etchrate.

[0016] If the electrode layer 24 is formed in this situation, theelectrode layer 24 intruded into the shape defect portion 28 will comeinto contact with the GaAs substrate 12 to cause short circuit. Suchshape defect portion 28 can be produced in the stack-structured portionmade of compound semiconductor layers not only during the wet etchingbut also during acid cleaning or alkali cleaning based on the samemechanism as described above.

[0017] As a result, short circuit will occur between currents injectedto the both electrodes, thereby current which essentially has to beinjected to the oscillation area in the active layer responsible forlaser oscillation is reduced, and it causes non-emissive failures suchthat no laser oscillation occurs or the laser oscillation does notcontinue.

[0018] It is, however, quite difficult in practice in fabricating thesemiconductor laser device to epitaxially grow the compoundsemiconductor layer after thoroughly cleaning the GaAs substrate andconfirming that no particles adhering thereon. Thus so long as thesemiconductor laser device is fabricated according to the conventionalprocess, those suffering from non-emissive failures will be more or lessproduced to degrade the production yield.

SUMMARY OF THE INVENTION

[0019] It is therefore an object of the present invention to provide asemiconductor laser device having a structure capable of preventingnon-emissive failure and a method for fabricating such device.

[0020] To accomplish such object, a semiconductor laser devicecomprises: a compound semiconductor substrate; a lower cladding layer;an active layer; an upper cladding layer and a contact layerrespectively formed on the compound semiconductor substrate, wherein anupper part of the upper cladding layer and the contact layer are formedas a mesa-structured portion having a ridge stripe pattern; and acurrent blocking layer having a pit-like recess penetrating thereof andextending towards the compound semiconductor substrate, the both sidesof the mesa structured portion are buried with the current blockinglayer, and a portion of the recess other than that penetrating thecurrent blocking layer being covered or buried with an insulating filmor a compound semiconductor layer with a high resistivity.

[0021] In the present invention, of the pit-like recess, a portion ofwhich other than that penetrating the current blocking layer is coveredor buried with an insulating film or a compound semiconductor layer witha high resistivity, so that the compound semiconductor substrate and theelectrode layers other than the a current injection area are keptinsulated, thereby the non-emissive failures as observed for theconventional semiconductor laser device is avoided.

[0022] The pit-like recess may not necessarily reach the compoundsemiconductor substrate and may be such that penetrating the currentblocking layer to reach the upper cladding layer, active layer or lowercladding layer. It is also allowable that not only a portion of therecess other than that penetrating the current blocking layer, but alsothe entire part of the recess is covered or buried with an insulatingfilm or a compound semiconductor layer with a high resistivity.

[0023] The current blocking layer is made of a compound semiconductorlayer with a high resistivity, or a current blocking layer using a p-njunction isolation.

[0024] The present invention is applicable irrespective of compositionsof the compound semiconductor substrate or compound semiconductor layersand, for example, preferably applicable to a semiconductor laser devicewith a laser oscillating structure composed of an AlGaInP-base orGaInP-base compound semiconductor layer formed on a GaAs substrate. Thepresent invention is applicable to both semiconductor laser devices ofedge-emitting type and surface-emitting type.

[0025] A structure responsible for the laser emission is not necessarilyof the stacked structure comprising the lower cladding layer, activelayer, upper cladding layer and contact layer, but also may be suchstructure that having a buffer layer between the substrate and the undercladding layer, or also may be such structure that having another layersuch as a light confining layer between the contact layer and uppercladding layer.

[0026] In a preferred embodiment of the present invention, theinsulating film may be made of at least any one of SiO₂ film, Al₂O₃ filmand SiN film, a thickness of which being within a range from 100 nm to50 μm. The insulating film may be a stacked film thereof.

[0027] The insulating film may be made of a semi-insulating materialdoped or ion-implanted with boron. The compound semiconductor layer witha high resistivity may be made of a GaAs layer with a low carrierdensity of, for example, from 1×10¹⁶/cm³ to 1×10¹⁸/cm³, both inclusive.

[0028] One method for fabricating such semiconductor laser device(referred as a first inventive method, hereinafter) relates to a methodfor fabricating a semiconductor laser device having on a compoundsemiconductor substrate at least a lower cladding layer, an activelayer, an upper cladding layer and a contact layer; an upper part of theupper cladding layer and the contact layer being formed as a mesastructured portion having a ridge stripe pattern, and the both side ofthe mesa structured portion being buried with a current blocking layer,the method comprises steps of:

[0029] forming a stacked structure on a compound semiconductor substrateby epitaxially growing thereon a lower cladding layer, an active layer,an upper cladding layer and a contact layer in this order,

[0030] forming an insulating film on the entire surface of the substrateincluding the wall plane of a pit-like recess penetrating the currentblocking layer and extending towards the compound semiconductorsubstrate,

[0031] forming a photoresist film on the entire surface of thesubstrate;

[0032] patterning the photoresist film to form a resist mask on theinsulating film as well as to fill the pit-like recess with thephotoresist film,

[0033] etching the insulating film using the resist mask as an etchingmask to form an insulating film mask, and then etching the contact layerand the upper cladding layer using the insulating film mask as anetching mask to form a mesa-structured portion having a ridge stripepattern,

[0034] selectively growing, using the insulating film mask as a mask, acurrent blocking layer thereby to bury the both sides of themesa-structured portion, and

[0035] removing the insulating film mask to expose the contact layer,and then forming an electrode layer on the surface of the substrateincluding on the contact layer.

[0036] Another method for fabricating such semiconductor laser device(referred as a second inventive method, hereinafter) relates to a methodfor fabricating a semiconductor laser device of an edge-emitting typehaving on a compound semiconductor substrate a lower cladding layer, anactive layer, an upper cladding layer and a contact layer; an upper partof the upper cladding layer and the contact layer being formed as a mesastructured portion having a ridge stripe pattern, and the both side ofthe mesa structured portion being buried with a current blocking layer,the method comprises steps of:

[0037] forming a stacked structure on a compound semiconductor substrateby epitaxially growing thereon a lower cladding layer, an active layer,an upper cladding layer and a contact layer in this order,

[0038] etching the contact layer and the upper cladding layer to form amesa-structured portion having a ridge stripe pattern,

[0039] selectively growing, using an insulating film mask, a currentblocking layer thereby to bury the both sides of the mesa-structuredportion,

[0040] removing the insulating film mask to expose the contact layer,and then forming an electrode layer on the surface of the substrate,

[0041] forming an insulating film on the entire surface of the substrateincluding the wall plane of a pit-like recess penetrating the currentblocking layer and extending towards the compound semiconductorsubstrate, and then removing the insulating film from an area other thanthe wall plane of the pit-like recess, and

[0042] forming an electrode layer on the surface of the substrateincluding on the contact layer.

[0043] Still another method for fabricating such semiconductor laserdevice (referred as a third inventive method, hereinafter) relates to amethod for fabricating a semiconductor laser device of an edge-emittingtype having on a compound semiconductor substrate a lower claddinglayer, an active layer, an upper cladding layer and a contact layer; anupper part of the upper cladding layer and the contact layer beingformed as a mesa structured portion having a ridge stripe pattern, andthe both side of the mesa structured portion being buried with a currentblocking layer, the method comprises steps of:

[0044] forming a stacked structure on a compound semiconductor substrateby epitaxially growing thereon a lower cladding layer, an active layer,an upper cladding layer and a contact layer in this order,

[0045] etching the contact layer and the upper cladding layer to form amesa-structured portion having a ridge stripe pattern,

[0046] selectively growing, using an insulating film mask, a currentblocking layer with a low carrier density thereby to bury the both sidesof the mesa-structured portion and a pit-like recess extending towardsthe compound semiconductor substrate, and then removing the insulatingfilm mask to expose the contact layer, and

[0047] forming an electrode layer on the surface of the substrateincluding the contact layer.

[0048] Still further another method for fabricating such semiconductorlaser device (referred as a fourth inventive method, hereinafter)relates to a method for fabricating a semiconductor laser device of anedge-emitting type having on a compound semiconductor substrate a lowercladding layer, an active layer, an upper cladding layer and a contactlayer; an upper part of the upper cladding layer and the contact layerbeing formed as a mesa structured portion having a ridge stripe pattern,and the both side of the mesa structured portion being buried with acurrent blocking layer, the method comprises steps of:

[0049] forming a stacked structure on a compound semiconductor substrateby epitaxially growing thereon a lower cladding layer, an active layer,an upper cladding layer and a contact layer in this order,

[0050] etching the contact layer and the upper cladding layer to form amesa-structured portion having a ridge stripe pattern,

[0051] selectively growing, using an insulating film mask, a currentblocking layer thereby to bury the both sides of the mesa-structuredportion, and then removing the insulating film mask to expose thecontact layer,

[0052] forming a resist pattern on the contact layer, and performing ionimplantation to the entire surface of the substrate thereby to convertthe outermost surface of the wall plane of a pit-like recess penetratingthe current blocking layer and extending towards the compoundsemiconductor substrate into a layer with a higher resistivity, and

[0053] removing the resist pattern thereby to form an electrode layer onthe surface of the substrate including on the contact layer withoutannealing.

[0054] While there is no specific limitation on a method for forming theinsulating film in the first to fourth inventive methods, the film ispreferably formed by the chemical vapor deposition (CVD) process. Thecurrent blocking layer is formed by the metal-organic chemical vapordeposition (MOCVD) process.

[0055] There is no specific limitation on ion species in the fourthinventive method, and boron can be ion-implanted for example.

[0056] In the first, second and fourth inventive methods, the wall planeof the pit-like recess conceptually include a bottom plane of therecess, as well as a side plane thereof.

[0057] According to the present invention, in the process of fabricatingthe semiconductor laser device, at least a portion excluding such thatpenetrating the current blocking layer of the pit-like recess, occurredso as to penetrate the current blocking layer and to reach the compoundsemiconductor substrate, is covered or filled with the insulating filmor the compound semiconductor layer with a higher resistivity, so thatthe compound semiconductor substrate and the electrode layer can be keptinsulated in an area other than a current injection area, therebynon-emissive failure as has been observed in the conventionalsemiconductor laser device is preventted.

[0058] The method according to the present invention embodies apreferable method for fabricating the semiconductor laser device of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0059]FIGS. 1a to 1 c are sectional views showing layer structurescorresponded to the individual process steps for fabricating asemiconductor laser device according to Example 1;

[0060]FIGS. 1d to 1 f are sectional views showing, as continued fromFIG. 1c, layer structures corresponded to the individual process stepsfor fabricating a semiconductor laser device according to Example 1;

[0061]FIGS. 2a to 2 c are sectional views showing layer structurescorresponded to the individual process steps for fabricating asemiconductor laser device according to Example 2;

[0062]FIGS. 2d and 2 e are sectional views showing, as continued fromFIG. 2c, layer structures corresponded to the individual process stepsfor fabricating a semiconductor laser device according to Example 2;

[0063]FIGS. 3a to 3 c are sectional views showing layer structurescorresponded to the individual process steps for fabricating asemiconductor laser device according to Example 3;

[0064]FIGS. 3d to 3 f are sectional views showing, as continued fromFIG. 3c, layer structures corresponded to the individual process stepsfor fabricating a semiconductor laser device according to Example 3;

[0065]FIGS. 4a to 4 c are sectional views showing layer structurescorresponded to the individual process steps for fabricating asemiconductor laser device according to Example 4;

[0066]FIG. 4d is a sectional view showing, as continued from FIG. 4c,layer structures corresponded to a process step for fabricating asemiconductor laser device according to Example 4;

[0067]FIG. 5 is a sectional view of a substrate showing a conventionalvisible light semiconductor laser device; and

[0068]FIG. 6 is a sectional view of a substrate for explaining thepit-like recess.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0069] Exemplary embodiments of the present invention will be explainedspecifically and in detail referring to the attached drawingshereinafter.

EXAMPLE 1

[0070] This Example relates to one embodiment of a semiconductor laserdevice of the present invention, and such laser device that obtained bya fabrication method according to the first inventive method. FIGS. 1ato 1 c, and FIGS. 1d to 1 f are sectional views showing layer structurescorresponded to the individual process steps for fabricating asemiconductor laser device according to this Example.

[0071] In a semiconductor laser device 38 of Example 1, as shown in FIG.1f, a pit-like recess 30 is formed penetrating an n-GaAs layer 22(current blocking layer) to reach a GaAs substrate 12, and a wall planecorresponded to a portion of the recess penetrating an upper claddinglayer 18, an active layer 16 and a lower layer 14 is covered with aninsulating film 32.

[0072] According to the semiconductor laser device 38 of Example 1,short circuit is avoided since the pit-like recess 30 is covered withthe insulating film 32, and such insulating film 32 is eventuallyinterposed between an electrode layer 37 and the GaAs substrate 12. Forthis, the GaAs substrate 12 and the electrode layer 37 can be keptinsulated in an area other than a current injection area, therebynon-emissive failure as has been observed in the conventionalsemiconductor laser device is prevented.

[0073] Next, a method for fabricating the semiconductor laser device 38according to Example 1 will be explained referring to FIGS. 1a to 1 f.

[0074] In this Example at first, similarly to the conventional process,the lower cladding layer 14 made of n-AlGaInP, the active layer 16, theupper cladding layer 18 made of p-AlGaInP and the contact layer 20 madeof p-GaAs are epitaxially grown on the GaAs substrate 12 in this orderby, for example, the metal-organic chemical vapor deposition (MOCVD)process to form the stacked structure, as shown in FIG. 1a.

[0075] Cleaning of such stacked structure using acid or alkali may insome cases result in formation of the pit-like recess 30 of several totens μm diameter penetrating the contact layer 20, upper cladding layer18, active layer 16 and lower cladding layer 14 and reaching the GaAssubstrate 12, due to the above-described growth defect formed during theepitaxial growth, as shown in FIG. 1a.

[0076] Therefore in the present Example, the insulating film 32 of 50 μmthick made of SiO₂ film, Al₂O₃ film or SiN film for forming a mask forthe selective growth is formed on the entire surface of the substrate soas to cover also inner surface of the pit-like recess 30 as shown inFIG. 1b.

[0077] Next, a photoresist film 34 is formed on the insulating film 32and is then processed to form a resist mask 34 for patterning theinsulating film 32 as shown in FIG. 1c. At this time, also the pit-likerecess 30 is filled with the photoresist film 34 as shown in FIG. 1c.

[0078] The insulating film 32 is then patterned using the resist mask 34so as to leave the insulating film 32 in the pit-like recess 30 as wellas to form an insulating film mask 36 used for the etching and selectivegrowth as shown in FIG. 1d.

[0079] After the resist mask 34 is removed, the contact layer 20 andupper cladding layer 18 are etched by the wet etching process using amixed solution of sulfuric acid and hydrogen peroxide as an etchant toform the mesa-structured portion having a ridge stripe pattern.

[0080] The n-GaAs layer 22 as a current blocking layer is thenselectively grown by the selective growth process using the insulatingfilm mask 36 as shown in FIG. 1e.

[0081] The insulating film mask 36 formed on the contact layer 20 andused for the selective growth is then removed, and the electrode layer37 serves as a p-side electrode is formed as shown in FIG. 1f. Thus thesemiconductor laser device 38 can be fabricated according to the presentExample.

EXAMPLE 2

[0082] This Example relates to another embodiment of a semiconductorlaser device of the present invention, and such laser device thatobtained by a fabrication method according to the second inventivemethod. FIGS. 2a to 2 c, and FIGS. 2d and 2 e are sectional viewsshowing layer structures corresponded to the individual process stepsfor fabricating a semiconductor laser device according to this Example.

[0083] In a semiconductor laser device 48 of Example 2, as shown in FIG.2e, a wall plane of a pit-like recess 40, occurred so as to penetratethe n-GaAs layer 22 (current blocking layer), upper cladding layer 18,active layer 16 and lower layer 14, and to reach the GaAs substrate 12,is covered with an insulating film 42.

[0084] According to the semiconductor laser device 48 of Example 2,short circuit is avoided since the pit-like recess 40 is covered withthe insulating film 42, and such insulating film 42 is eventuallyinterposed between an electrode layer 46 and the GaAs substrate 12.

[0085] For this, the GaAs substrate 12 and the electrode layer 46 can bekept insulated in an area other than a current injection area, therebynon-emissive failure as has been observed in the conventionalsemiconductor laser device is prevented.

[0086] Next, a method for fabricating the semiconductor laser device 48according to Example 2 will be explained referring to FIGS. 2a to 2 e.

[0087] In this Example at first, similarly to the conventional process,the lower cladding layer 14 made of n-AlGaInP, the active layer 16, theupper cladding layer 18 made of p-AlGaInP and the contact layer 20 madeof p-GaAs are epitaxially grown on the GaAs substrate 12 in this orderby, for example, the metal-organic chemical vapor deposition (MOCVD)process to form the stacked structure, as shown in FIG. 2a.

[0088] Next, the contact layer 20 and upper cladding layer 18 are etchedby the wet etching process using a mixed solution of sulfuric acid andhydrogen peroxide as an etchant to form the mesa-structured portion, andthe n-GaAs layer 22 is then selectively grown on the upper claddinglayer 18 and on the both sides of the mesa-structured portion.

[0089] In the stacked structure thus processed, the pit-like recess 40of several to tens μm diameter may in some cases occur so as topenetrate the n-GaAs layer 22, upper cladding layer 18, active layer 16and lower cladding layer 14 and reaching the GaAs substrate 12, due tothe above-described growth defect formed during the epitaxial growth, asshown in FIG. 2a.

[0090] Therefore in the present Example 2, the insulating film 42 of 50μm thick made of SiO₂ film or Al₂O₃ film is formed on the entire surfaceof the substrate so as to cover also inner surface of the pit-likerecess 40 as shown in FIG. 2b.

[0091] Next, a photoresist film 44 is formed on the entire surface ofthe substrate so as to also fill the pit-like recess 40, and thephotoresist film 44 is then removed in an area exclusive of the filledportion in the pit-like recess 40.

[0092] The insulating film 42 is then removed using the photoresist mask44 in an area exclusive of that corresponded to the pit-like recess 40thereby to expose the contact layer 20 and n-GaAs layer 22 as shown inFIG. 2d.

[0093] The photoresist film 44 is then removed, and an electrode layer46 serves as a p-side electrode is formed on the entire surface of thesubstrate as shown in FIG. 2e. Thus the semiconductor laser device 48can be fabricated according to the present Example.

EXAMPLE 3

[0094] This Example relates to still another embodiment of asemiconductor laser device of the present invention, and such laserdevice that obtained by a fabrication method according to the thirdinventive method. FIGS. 3a to 3 c, and FIGS. 3d to 3 f are sectionalviews showing layer structures corresponded to the individual processsteps for fabricating a semiconductor laser device according to thisExample.

[0095] In a semiconductor laser device 59 of Example 3, as shown in FIG.3f, a wall plane of a pit-like recess 50, occurred so as to penetratethe upper cladding layer 18, active layer 16 and lower layer 14, and toreach the GaAs substrate 12, is filled with the n-GaAs layer 22 with alow carrier density of, for example, 1×10¹⁸/cm³ or less.

[0096] According to the semiconductor laser device 59 of Example 3,short circuit is avoided since the pit-like recess 50 is filled with then-GaAs layer 22 with a low carrier density, and such n-GaAs layer 22 iseventually interposed between an electrode layer 58 and the GaAssubstrate 12.

[0097] For this, the GaAs substrate 12 and the electrode layer 58 can bekept insulated in an area other than a current injection area, therebynon-emissive failure as has been observed in the conventionalsemiconductor laser device is prevented.

[0098] Next, a method for fabricating the semiconductor laser device 59according to Example 3 will be explained referring to FIGS. 3a to 3 f.

[0099] In this Example at first, similarly to the conventional process,the lower cladding layer 14 made of n-AlGaInP, the active layer 16, theupper cladding layer 18 made of p-AlGaInP and the contact layer 20 madeof p-GaAs are epitaxially grown on the GaAs substrate 12 in this orderby, for example, the metal-organic chemical vapor deposition (MOCVD)process to form the stacked structure, as shown in FIG. 3a.

[0100] Cleaning of such stacked structure using acid or alkali may insome cases result in formation of the pit-like recess 50 of several totens μm diameter penetrating the contact layer 20, upper cladding layer18, active layer 16 and lower cladding layer 14 and reaching the GaAssubstrate 12, due to the above-described growth defect formed during theepitaxial growth, as shown in FIG. 3a.

[0101] Next, the insulating film 52 of 50 μm thick made of SiO₂ film orAl₂O₃ film for forming a mask for the etching and selective growth isformed on the entire surface of the substrate as shown in FIG. 3b.

[0102] The photoresist film 54 is then formed on the insulating film 52and is then processed to form a resist mask 54 for patterning theinsulating film 52 as shown in FIG. 3c.

[0103] At this time in Example 3, photoresist film 54 is etched based onetching conditions not allowing the photoresist film 54 remain in thepit-like recess 50, unlike Example 1.

[0104] The insulating film 52 is then etched using the photoresist mask54 to form an insulating film mask 56 as shown in FIG. 3d.

[0105] Next, the contact layer 20 and upper cladding layer 18 are etchedusing the insulating film mask 56 to form the mesa-structured portion,and the n-GaAs layer 22 with a low carrier density of, for example,1×10¹⁸/cm³ or below is then grown by the selective growth process usingthe mask 56 to fill the both sides of the mesa-structured portion andthe pit-like recess 50 as shown in FIG. 3e.

[0106] The insulating film mask 56 is then removed to expose the contactlayer 20, and the electrode layer 58 serves as a p-side electrode isthen formed on the n-GaAs layer 22 and contact layer 20 by, for example,the sputtering process. Thus the semiconductor laser device 59 can befabricated according to the present Example.

EXAMPLE 4

[0107] This Example relates to still further another embodiment of asemiconductor laser device of the present invention, and such laserdevice that obtained by a fabrication method according to the fourthinventive method. FIGS. 4a to 4 c are sectional views showing layerstructures corresponded to the individual process steps for fabricatinga semiconductor laser device according to this Example.

[0108] In a semiconductor laser device 66 of Example 4, as shown in FIG.4d, a outermost surface of a wall plane of a pit-like recess 40,occurred so as to penetrate the n-GaAs layer 22 (current blockinglayer), upper cladding layer 18, active layer 16 and lower layer 14, andto reach the GaAs substrate 12, is converted into a layer 62 with ahigher resistivity by ion implantation of boron.

[0109] According to the semiconductor laser device 66 of Example 4,short circuit is avoided since the layer 62 having a higher resistivityis provided between the an electrode layer 64 and the GaAs substrate 12.For this, the GaAs substrate 12 and the electrode layer 64 can be keptinsulated in an area other than a current injection area, therebynon-emissive failure as has been observed in the conventionalsemiconductor laser device is prevented.

[0110] Next, a method for fabricating the semiconductor laser device 66according to Example 4 will be explained referring to FIGS. 4a to 4 d.

[0111] In this Example at first, similarly to Example 2, the lowercladding layer 14 made of n-AlGaInP, the active layer 16, the uppercladding layer 18 made of p-AlGaInP and the contact layer 20 made ofp-GaAs are epitaxially grown on the GaAs substrate 12 in this order by,for example, the metal-organic chemical vapor deposition (MOCVD) processto form the stacked structure, as shown in FIG. 4a.

[0112] Next, the contact layer 20 and upper cladding layer 18 are etchedby the wet etching process using a mixed solution of sulfuric acid andhydrogen peroxide as an etchant to form the mesa-structured portion, andthe n-GaAs layer 22 is then selectively grown on the upper claddinglayer 18 and on the both sides of the mesa-structured portion.

[0113] In this stage after such processes are completed, the pit-likerecess 40 of several to tens μm diameter may in some cases occur so asto penetrate the n-GaAs layer 22, upper cladding layer 18, active layer16 and lower cladding layer 14 and reaching the GaAs substrate 12, dueto the above-described growth defect formed during the epitaxial growth,as shown in FIG. 4a.

[0114] Therefore in the Example 4, a photoresist film is formed on theentire surface of the substrate, and the film is then patterned to forma resist mask 60 covering at least the contact layer 20 while exposingan area including the pit-like recess 40 as shown in FIG. 4b.

[0115] Ions, for example boron ions, are then implanted according to theconditions shown below using the resist mask 60 as a mask as shown inFIG. 4c thereby to convert the outermost surfaces of the GaAs substrate12, lower cladding layer 14, active layer 16, upper cladding layer 18and n-GaAs layer 22 into a layer 62 with a higher resistivity:

[0116] Implantation energy: 140 keV

[0117] Dose amount: 7×10¹⁴/cm²

[0118] The resist mask 60 is then removed to expose the contact layer20, and the electrode layer 64 serves as a p-side electrode is thenformed on the n-GaAs layer 22 and contact layer 20 by, for example, thesputtering process without performing annealing for the ion implantedsurfaces. Thus the semiconductor laser device 66 can be fabricatedaccording to the present Example.

What is claimed is:
 1. A semiconductor laser device comprising: acompound semiconductor substrate; a lower cladding layer; an activelayer; an upper cladding layer and a contact layer respectively formedon the compound semiconductor substrate, wherein an upper part of theupper cladding layer and the contact layer are formed as amesa-structured portion having a ridge stripe pattern; and a currentblocking layer having a pit-like recess penetrating thereof andextending towards the compound semiconductor substrate, the both sidesof the mesa structured portion being buried with the current blockinglayer, and a portion of the recess other than that penetrating thecurrent blocking layer being covered or buried with an insulating filmor a compound semiconductor layer with a high resistivity.
 2. Asemiconductor laser device as claimed in claim 1, wherein the insulatingfilm is made of at least any one of SiO₂ film, Al₂O₃ film and SiN film.3. A semiconductor laser device as claimed in claim 1, wherein theinsulating film is made of a semi-insulating material doped orion-implanted with boron.
 4. A semiconductor laser device as claimed inclaim 1, wherein the compound semiconductor layer with a highresistivity is made of a GaAs layer with a low carrier density.
 5. In amethod for fabricating a semiconductor laser device having on a compoundsemiconductor substrate at least a lower cladding layer, an activelayer, an upper cladding layer and a contact layer; an upper part of theupper cladding layer and the contact layer being formed as a mesastructured portion having a ridge stripe pattern, and the both side ofthe mesa structured portion being buried with a current blocking layer,the method comprising steps of: forming a stacked structure on acompound semiconductor substrate by epitaxially growing thereon a lowercladding layer, an active layer, an upper cladding layer and a contactlayer in this order, forming an insulating film on the entire surface ofthe substrate including the wall plane of a pit-like recess penetratingthe current blocking layer and extending towards the compoundsemiconductor substrate, forming a photoresist film on the entiresurface of the substrate, patterning the photoresist film to form aresist mask on the insulating film as well as to fill the pit-likerecess with the photoresist film, etching the insulating film using theresist mask as an etching mask to form an insulating film mask, and thenetching the contact layer and the upper cladding layer using theinsulating film mask as an etching mask to form a mesa-structuredportion having a ridge stripe pattern, selectively growing, using theinsulating film mask as a mask, a current blocking layer thereby to burythe both sides of the mesa-structured portion, and removing theinsulating film mask to expose the contact layer, and then forming anelectrode layer on the surface of the substrate including on the contactlayer.
 6. In a method for fabricating a semiconductor laser device of anedge-emitting type having on a compound semiconductor substrate a lowercladding layer, an active layer, an upper cladding layer and a contactlayer; an upper part of the upper cladding layer and the contact layerbeing formed as a mesa structured portion having a ridge stripe pattern,and the both side of the mesa structured portion being buried with acurrent blocking layer, the method comprising steps of: forming astacked structure on a compound semiconductor substrate by epitaxiallygrowing thereon a lower cladding layer, an active layer, an uppercladding layer and a contact layer in this order, etching the contactlayer and the upper cladding layer to form a mesa-structured portionhaving a ridge stripe pattern, selectively growing, using an insulatingfilm mask, a current blocking layer thereby to bury the both sides ofthe mesa-structured portion, removing the insulating film mask to exposethe contact layer, and then forming an electrode layer on the surface ofthe substrate, forming an insulating film on the entire surface of thesubstrate including the wall plane of a pit-like recess penetrating thecurrent blocking layer and extending towards the compound semiconductorsubstrate, and then removing the insulating film from an area other thanthe wall plane of the pit-like recess, and forming an electrode layer onthe surface of the substrate including on the contact layer.
 7. In amethod for fabricating a semiconductor laser device of an edge-emittingtype having on a compound semiconductor substrate a lower claddinglayer, an active layer, an upper cladding layer and a contact layer; anupper part of the upper cladding layer and the contact layer beingformed as a mesa structured portion having a ridge stripe pattern, andthe both side of the mesa structured portion being buried with a currentblocking layer, the method comprising steps of: forming a stackedstructure on a compound semiconductor substrate by epitaxially growingthereon a lower cladding layer, an active layer, an upper cladding layerand a contact layer in this order, etching the contact layer and theupper cladding layer to form a mesa-structured portion having a ridgestripe pattern, selectively growing, using an insulating film mask, acurrent blocking layer with a low carrier density thereby to bury theboth sides of the mesa-structured portion and a pit-like recessextending towards the compound semiconductor substrate, and thenremoving the insulating film mask to expose the contact layer, andforming an electrode layer on the surface of the substrate including thecontact layer.
 8. In a method for fabricating a semiconductor laserdevice of an edge-emitting type having on a compound semiconductorsubstrate a lower cladding layer, an active layer, an upper claddinglayer and a contact layer; an upper Dart of the upper cladding layer andthe contact layer being formed as a mesa structured portion having aridge stripe pattern, and the both side of the mesa structured portionbeing buried with a current blocking layer, the method comprising stepsof: forming a stacked structure on a compound semiconductor substrate byepitaxially growing thereon a lower cladding layer, an active layer, anupper cladding layer and a contact layer in this order, etching thecontact layer and the upper cladding layer to form a mesa-structuredportion having a ridge stripe pattern, selectively growing, using aninsulating film mask, a current blocking layer thereby to bury the bothsides of the mesa-structured portion, and then removing the insulatingfilm mask to expose the contact layer, forming a resist pattern on thecontact layer, and performing ion implantation to the entire surface ofthe substrate thereby to convert the outermost surface of the wall planeof a pit-like recess penetrating the current blocking layer andextending towards the compound semiconductor substrate into a layer witha higher resistivity, and removing the resist pattern thereby to form anelectrode layer on the surface of the substrate including on the contactlayer without annealing.